Photomasks, which contain image areas opaque to actinic radiation are known in the field of photographic reproduction as originals for use in the imagewise exposure of light sensitive material, for example for the production of color separations, printing plates, etc.
Silver halide films, known as "litho films" are used for the preparation of halftone photomasks in which the image is formed of dots of various sizes. It is often necessary for the tonal values of the developed silver image to be corrected. For this purpose, the halftone dots are reduced by a dot etching process, in which the silver present in the exposed image areas is partially removed oxidatively by a silver dissolving agent. A disadvantage of this type of silver halide film is its high silver coating weight and consequently, its high production cost.
It is known that exposed photographic materials, which contain an unhardened or only slightly hardened silver halide/gelatin emulsion layer, can be hardened imagewise with tanning developers. The tanning developer oxidation products produced by development in the exposed image areas have the property of hardening gelatin, In the unexposed image areas Where development does not occur, there is no hardening and therefore, these image areas can either be washed off with the formation of a relief image or transferred wholly or partially onto an image receptor. The addition of pigments or dyes, particularly carbon black, to the light sensitive material is known, in order to confer adequate optical density to the image areas. This makes it possible to use less silver halide than is needed for image production. These wash-off films are described, for example, in the following patents and patent application: U.S. Pat Nos. 2,596,756, 3,364,024, 3,440,049, 4,233,392, 4,369,245, 4,427,757 and 4,504,572 and Bristish Patent No. 1,294,355.
An important disadvantage of the wash-of films for tanning development, compared to (conventional) silver halide films, is their lack of wet etchability, which limits significantly their use in reprography. This applies particularly in the preparation and reproduction of color separations for multicolor printing.
In principle this problem should be solvable by the use of materials with colloidal silver as the image-forming pigment, as are described, for example, in European Patent Application EP 01 95 327. because such films should be etchable by the conventional methods. However, the use of such films involves serious technical process concerns, because, as unhardened film areas are washed off, silver enters the wash water, Solving this Problem involves not only high technology but is expensive. Another disadvantage is the higher film material cost, which results from the higher price of the silver pigment compared to other pigments.
Many other pigments and dyes are unsuitable, because they affect adversely the imaging ability of the silver halide or the tanning process. Compounds that tend to crystallize or "bloom" in the film layers are not suitable as pigments or dyes. This also applies to materials that are bleached by the action of the processing baths or are dissolved out of the film layer. However, it should be possible, after processing, to bleach or partially remove the pigments or dyes from the film, if desired, to achieve a reduction of the halftone dots. A dot reduction without a decrease in optical density is most desirable. In addition, the mechanical stability of the film should remain unchanged, and the film should not be stained by the eliminated pigments or dyes.
Therefore, the present invention is directed to a system for the preparation of dot-etched lithographic films that contain pigments other than silver and that do not adversely affect the advantages of wash-off film systems, such as low silver coating weight and fast, automatic processing,